Abstract Novel therapeutic strategies such as modulation of dendritic cell and T cell functions have shown great potential in clinical transplantation. HLA-G is a molecule that plays a significant role in establishing complex mechanisms to protect semi-allogeneic fetuses from rejection by the maternal immune system. The tolerogenic potential of HLA-G is mediated by its exclusive binding to the inhibitory receptors on immunocompetent cells that modulate cell functions. However, the mechanisms by which HLA-G modulates the function of dendritic cells and T cells are thus far incompletely understood. The unique characteristics of both cell surface and soluble isoforms of HLA-G, the formation of disulfide-bonded dimers with the potential to augment inhibitory receptor signaling, and the function of HLA-G as a preferential ligand for the immunoglobulin-like transcript receptors make HLA-G very important in fundamental approaches for modulation of immune responses to improve allogeneic graft survival in clinical transplantation. This application will address the cellular and molecular mechanisms of tolerization of dendritic cells and the expansion and enhancement function of myeloid-derived suppressor cells mediated by HLA-G, leading to the prolongation of allograft survival. The long-term goal of this research is to determine the mechanisms of modulation of dendritic cells, myeloid- derived suppressor cells, and T cells by HLA-G, and to design a highly potent HLA-G-based immunotherapy against allograft rejection. We have developed models to determine the mechanisms of tolerogenic function of HLA-G in vitro and in vivo. The central hypothesis of this application, which is based on our published and strong preliminary data from experiments involving HLA-G-mediated human tolerogenic dendritic cells in vitro and receptor transgenic mice in vivo, is that different isoforms of HLA-G have various immunomodulatory effects through the inhibitory receptors, and that knowledge of this can be crucial in designing the most potent form of HLA-G. This hypothesis is reflected in the study of the mechanism of tolerization of dendritic cells (Specific Aim 1) and mechanisms of prolongation of allograft survival mediated by HLA-G and inhibitory receptors on myeloid-derived suppressor cells (Specific Aim 2). Data obtained from these studies will reveal the potential of HLA-G in modulation of immune responses and will aid in development of novel strategies for translation into the clinic to improve allograft survival in patients and treat graft-versus-host disease, allergy, and autoimmune diseases